Research

Research in the Harrington lab focuses on mosquito vector ecology, biology, and behavior. Our goal is to understand basic (and often overlooked) aspects of mosquito biology in order to identify new targets for controlling mosquitoes and reducing transmission of vector-borne diseases. We examine long-standing questions in the field of vector biology from broad and, sometimes unconventional, angles. Research in the Harrington lab focuses on the most dangerous mosquito vectors impacting human health today; those that transmit dengue, yellow fever and West Nile viruses and those that transmit malaria parasites. We use a combined approach to studying disease vectors that includes biochemical and proteomics tools as well as conducting classical field studies. This approach allows us to validate laboratory findings in epidemiologically-relevant field settings leading to more meaningful research outcomes.

Harmonic convergence and acoustic biology of mosquitoes. For decades scientists have known that male mosquitoes identify conspecific females

Aedes aegypti in copula. Photo by Charles Walcott.

based on their fundamental flight tones. However, females were considered deaf or unresponsive to male sounds. In addition, scientist believed that the mosquito mating system was devoid of any sort of courtship behavior. In 2009, together with the Hoy lab, we discovered a pre-copulatory courtship phenomenon in Ae. aegypti mosquitoes which we termed “harmonic convergence”. We have now demonstrated this behavior, where male and female mosquitoes alter their flight tones prior to mating so they match at harmonic frequencies, (Cator et al. 2009; Cator et al. 2011; Arthur et al. 2010) in An. gambiae (Cator et al. 2010); other researchers have demonstrated that it is likely to be common across the mosquito taxa. The exact function of harmonic convergence is not clear, but it is probably a component of mate assessment. We have found that successful convergence between pairs of Ae. aegypti significantly predicts the formation of a successful copula (Cator and Harrington, 2011). In another study with the malaria vector, An. gambiae, we used computer generated acoustic playback to determine the effect of flight tone frequency (which is correlated with body size) on harmonic convergence interactions (Cator et al. 2010). We found that males and females altered components of convergence behavior depending on the body size of a perceived potential mate. Further work in the Harrington lab has demonstrated that this behavior is associated with increased mating success in sons and that harmonic convergence is heritable (Cator and Harrington, 2011). Our exploration of the information convergence is signaling is ongoing with an NIH-funded project let by Dr. Lauren Cator, now at Imperial College and collaborations with Dr. Courtney Murdock at the University of Georgia.

Additional work has focused on use of fundamental flight tone for male attraction. With funding from the Bill and Melinda Gates Foundation, we developed a prototype male trap and demonstrated the importance of temperature on fundamental flight tone frequency (Villarreal et al. 2017).

Watch a video of mating rejection and acceptance behaviors (video by L. Cator).

Biology of male Aedes seminal fluid proteins and sperm. In other insects, male seminal fluid proteins that are transferred to the female during copulation can have significant and essential impacts on female physiology and behavior. Relatively little was known about whether similar molecules exist in the mosquito mating system and, if so, whether they could be used as new tools that target mosquito control reducing vector populations and breaking the cycle of dengue transmission. We began to investigate this area with a bioinformatics and proteomic based study and identified 63 putative sfps in Ae. aegypti (Sirot et al. 2008). In a second study we used a novel “reverse isotope labeling” approach to track and determine transfer of sfps from males to females. With this approach we identified 93 male-derived Sfps and 52 predicted sperm proteins that are transferred to females during mating (Sirot et al. 2011). The Sfp protein classes we detected suggest intriguing and potentially critical roles of male sfps on females including protein activation/ inactivation, induction of host seeking and blood feeding, sperm utilization, and ecdysteroidogenesis. Recently, we investigated and published our findings for male sfps in Ae albopictus (Boes et al. 2014). In addition, we’ve developed behavioral assays to evaluate the impact of individual sfp molecules on female blood feeding, re-mating refractoriness, oviposition and survival. We also have examined the duration and dose-dependency of female sexual receptivity responses to seminal fluid proteins (Helinski et al. 2012). We also determined the fate and transfer of one sfp (AAEL010824) in detail (Alfonso-Parra et al. 2014). We showed that AAEL010824 is first detectable by 12 h post-eclosion, and increases in amount over the first 3 days of adult life. We then showed that the amount of AAEL0010824 in the AG decreases after mating, with each successive mating depleting it further; by 5 successive matings with no time for recovery, its levels are very low. AAEL010824 levels in a depleted male are replenished by 48 h post-mating. In addition we made a transgenic mosquito line that carries EGFP fused to the AAEL0010824 promoter. We showed that AAEL010824 is expressed in the anterior cells of the accessory glands, and that its RNA levels also respond to mating. We intend to use this promoter in the future for driving male AG expression of genes of interest.

Recently we created the first mating-induced transcriptome (Alfonso-Parra et al. 2016). A total of 150 transcripts were significantly up-regulated in Ae. aegypti and 130 were down-regulated after mating. A small subset of these transcripts is likely male-derived: their abundance in females increased dramatically within 20 seconds after mating (and then decreased).

Graduate student Ethan Degner’s dissertation work has focused on the biology of mosquito sperm. Recently we reviewed the literature on this topic and provided a synthesis and research framework for exploring the important role of mosquito sperm (Degner and Harrington 2016a). Ongoing work focuses on sperm structure and movement in the female reproductive tract.

Sampling malaria vectors at night, Tanzania

Mating behavior of mosquitoes in field and laboratory. We have made significant contributions to understanding the mosquito mating system of Ae aegypti in both the laboratory and field. For example, we’ve studied free flight acoustic interactions at field sites in Thailand (Cator et al. 2011), and we’ve assessed components of male quality and fitness with wild and transgenic males in the laboratory and field (Ponlawat and Harrington 2009; Helinski and Harrington 2011; Helinski and Harrington 2013). We also were the first to describe polyandry in this species in semi-field conditions (Helinski et al. 2012) and have refined our understanding of the timing of polyandry in the laboratory (Degner and Harrington 2016b). Our studies of mosquito mating biology and behavior continue across the spectrum from lab to field.

Mosquito blood feeding behavior. The act of ingesting a host blood meal is a critical step and potential target in pathogen transmission. We have investigated mosquito blood feeding patterns of important disease vectors at several scales. One of the early questions that we addressed was why Ae. aegypti feeds preferentially on human hosts when so many other hosts are available to it in its natural environment. What we learned is that the notion that human blood was reproductively sub-optimal for this species was completely the result of a lab artifact (sugar feeding, which is a rare event for Ae. aegypti across most of its natural range, Spencer et al. 2005). We then went on to demonstrate that human blood is both reproductively and energetically optimal for this species, imparting a significant fitness advantage to Ae. aegypti (Harrington et al. 2001). In addition, we have compared human blood feeding preferences for Ae. aegypti and Ae. albopictus across a transect of Thailand, where both species play a role in DENV transmission (Ponlawat and Harrington 2005). We found very high rates of human feeding and high forage ratios given censes of available hosts. This was surprising given the assumption that Ae. albopictus is considered a generalist feeder, thereby decreasing its vector potential for human pathogens. We also have investigated feeding on individual humans using polymorphic human microsatellite markers in the mosquito blood meal. In Thailand, we found that certain individuals were consistently fed on multiple times, with no clear host characteristics (sex, age, location) that may drive these patterns (Harrington et al. 2014). Other important outcomes of this work was the demonstration that the epidemiologically DENV naive hosts overall (those less than 25 years old) were fed on less frequently than older hosts. We also demonstrated a direct relationship with host body mass (height x weight) and mosquito feeding rate. We found similar trends with the malaria vector mosquitoes, An. funestus and An. gambiae in a focus of malaria transmission in Kenya (Scott et al. 2006). Other work in the Harrington laboratory has explored the role of host defensive behavior on mosquito blood feeding success, showing that feeding success can be reduced by host species and defensive movement, and that that preferential feeding on individual animals applies to avian hosts as well as humans (Darbro and Harrington 2007; Darbro et al. 2007).

Mosquito surveys in Medellin, Colombia.

Biology and transmission potential of the Asian Tiger Mosquito. Ae. albopictus is becoming increasingly more important as a vector of arboviruses and is well established as a transmitter of dog heartworm in the US (Ledesma and Harrington 2011; Brown et al. 2012; Ledesma et al. in press). Ae. albopictus is highly invasive and the factors that drive their invasion and establishment in new regions is unclear. Recently, we published a review (Shragai et al. 2017) highlighting current knowledge about the transmission ecology of Ae. albopictus focusing on viral biology, historical routes of transmission and viral mechanisms that facilitate its rapid global invasion.

We are currently investigating the ecology and overwintering biology of Ae. albopictus along the northern limit of its range in the Northeast USA. Studies are planned to investigate other aspects of the feeding behavior and movement. We have also demonstrated the vector competence of US Asian Tiger mosquitoes for emerging viruses, such as Chikungunya, and modelled potential outbreaks after introduction to the US (Ruiz-Moreno et al. 2012)